Oxide fine particles and composite oxide fine particles of elements such as zirconium, titanium and tin have a higher refractive index than silica particles and are generally used as fillers for controlling the refractive index of transparent coating films such as hardcoat films.
However, adding such high-refractive index particles to a film-forming coating composition results in problems that they exhibit a low reactivity with a binder component and poor dispersion stability in the coating composition or a film.
Meanwhile, coating compositions that contain an organosilicon compound or a resin composition as a binder component are often required to be stored or used while being in an acidic region. However, a dispersion liquid containing such high-refractive index particles is unstable in an acidic pH region and the particles are easily aggregated.
Surface treatment with a silane coupling agent is known as a remedy to increase the dispersibility of particles. However, it is difficult to adequately treat the surface of such particles with a silane coupling agent, often failing to achieve sufficient dispersion stability. Thus, there has been a need for a more effective treatment method.
Patent Literature 1 discloses a method for improving the dispersibility of particles by covering the surface of zirconia fine particles with antimony pentoxide and/or silica.
Titanium oxide fine particles or composite oxide fine particles have another problem in that the weather resistance and the light resistance of coating films are lowered due to the activity of titanium. To remedy this problem, Patent Literature 2 discloses composite oxide fine particles in which nuclear particles of an oxide of titanium and tin are covered with silicon oxide and an oxide of zirconium and/or aluminum.
Patent Literature 3 discloses a titanium oxide sol in which titanium oxide sol particles are covered with a layer formed of a hydrated oxide of silicon and aluminum.
Further, Patent Literature 4 discloses inorganic oxide fine particles in which a silica-alumina hydrate has been deposited on the surface. The particles are described as an intermediate in the production of antibacterial inorganic oxide fine particles.